This invention is a thermal printer which is suitable for high speed printing with high quality.
Thermal printers have come into widespread use in various types of printers including those incorporated in facsimile equipment for recording picture images. Conventional thermal printers have a number of heating resistors arranged in a row on a substrate. These resistors are cyclically heated by selectively supplying electric current according to picture data. An image is recorded on a heat-sensitive paper which faces the heating resistors while the paper is moved in the direction perpendicular to the resistor array. While this kind of thermal printer is characterized by absence of noise, clean recording and ease of maintenance, a less desirable feature has been the difficulty of raising the speed of printing due to the heat-storage effect of the heating resistors. If, that is to say, the duty cycle is shortened in order to achieve high speed, heat is accumulated in the resistors since electrical current is repeatedly applied to the resistors before the heat generated in the previous cycle has been dissipated, so that the temperature continues to rise. Since the amount of heat accumulated in the resistors is different for each one depending on the picture data, this leads to a lack of uniformity in printing density. Further, the fact that the heat of the previous cycle remains up to the next cycle can lead to darkening of the heat-sensitive paper in places where there are space data, that is, where there should be no such darkening, so that ghost images appear.
In order to solve this problem, a method has been proposed whereby, for each heating resistor, if mark data arrive continuously in the picture signal data, the duty cycle (current passage time or pulse width) is made shorter than if mark data arrive after space data (Japan Patent Publication 55-48631). Realizing the principle, the method requires at least five gate circuits for each heating resistor which may number as many as 1000 to 2000. The thermal printer according to the prior art has, therefore, defects in that it is complicated, costly and not compact.